Tribology is a science of friction, wear, and lubrication on friction surfaces. Many different types of strain-gauges and other devices for force measuring are known in the art.
U.S. Pat. No. 6,324,918 issued in 2001 to N. Gitis, at all, describes a bidirectional force sensor for measuring two forces applied in two non-parallel directions (FIG. 1). This sensor, which in general is designated by reference numeral 10, is used on a friction tester. The friction tester, which in FIG. 1 is represented by a sensor mounting plate 42, has an upper rod-like test material specimen or probe 44 and a lower disk-like test material specimen 46, which performs rotary motion in the direction indicated by arrow R, while being in contact with a stationary upper specimen 44. This sensor comprises a flexible beam 12 of a rectangular cross section with rigid solid end blocks 14 and 16 at both ends for securing the sensor in a tester. The beam has two symmetrically-shaped through slots cut in mutually perpendicular directions so that they are partially intersected within a body of the beam. Each slot has at its opposite ends notches which are wider than the slots so that the distance from the inner wall of the notch to the outer side surface of the beam is shorter than the distance to this surface from the inner wall of the slot. Strain gauges 38, 40 are attached to mutually perpendicular surfaces at the ends of the beam which are flexible in the direction of the force being measured and are rigid in the perpendicular direction. Under effect of the loading force F1 and of the friction force, the flexible beam acts as a pair of overlapped and mutually perpendicular parallelograms.
A disadvantage of this device is that the upper specimen has a leverage with respect to the point of attachment of the lower specimen, i.e., with respect to its center. As a result, the loading force applied to the lower specimen via the upper probe, as well as the reaction force applied to the probe from the lower specimen create an unbalanced momentum and deformations in the force measurement system.
U.S. Pat. No. 6,363,798 issued in 2002 to N. Gitis, at all, describes another device for measuring a loading force and a friction force in a tribological tester (FIG. 2). This device consists of two deformation-sensitive sensors 51 and 52 for simultaneous equal deformation in two opposite directions for eliminating misbalance created in the measurement system when a single sensor is used. Each sensor comprises a deformable beam 55, 56 having through longitudinal slots 58, 59 extending in different and non-parallel directions and overlapped within the body of the beam. The sensor deforms in one direction under the effect of a loading force 62 measured by two pairs of strain gauges 65, 66 located on opposite sides of the beam near one end of the beam and in another direction under the effect of a friction force 68 measured by another two pairs of strain gauges 70, 71 located on opposite sides of the beam near the other end of the beam. Two sensors are sandwiched between two plates 74, 75 in diagonally symmetrical positions so as to transmit forces between both plates and at the same time to ensure limited freedom of movement between both plates to allow deformations caused by the applied forces. One plate may be attached to the loading unit of the tester (not shown) and another plate may support an upper sample 80 for engagement with the lower sample 90 of the tester.
However, the above sensors have low torsional stability due to the fact that deformable beams in these sensors have elongated shape with reduced cross-sections in the deformable areas. In the tribometers and friction testers the samples for testing are usually mounted to the force sensor not directly but in special holders, which may have significant length (up to several inches) for placing the samples into a testing media (usually fluids or gases) or/and into environmental chambers (heating, cooling, pressure, humidity, etc.).
When a sample mounted in such a holder is brought in contact with moving counter-sample and a normal load is applied to press the samples together a friction force is developed in contact between two samples, which acts in the direction parallel to the direction of motion of a moving sample.
This friction force being applied to a sample mounted in a long holder creates a significant torsional moment in the force sensor directly proportional to the friction force magnitude and to the holder length.
The reduced cross-sections of the flexible beams result in reduction of the torsional stiffness of the force sensor, that causes significant torsional deformation of the force sensor sensitive elements, especially in case when the tangential (friction) force applied to sensor with an offset relative to the longitudinal axis of the sensor, as it usually takes place in friction testers and tribometers (where the force is applied to a specimen attached to the sensor via an elongated holder). This torsional deformation could be a source of additional error of the force measurement. Also, it results in a noticeable tilt of the specimen and specimen holder, increases cross-talk between the normal load and the friction force signals, adding to the measurement error and instability. Besides, under certain conditions, these torsional deformations in combination with moving contacting samples can cause such a parasitic effect as a vibration of the sensor assembly.